Thanks Paul, sorry about the delay… blame it on the weekend 
The focus distance is from the lens, so it’s position from the workspace can’t really be changed with a single focal length lens.
You can shorten the tube, that would allow more room between end of tube and material.
I use the C series tubes These allow you to flip them over for different distances to the material/lenses.
I have a 4" lens, and I believe you can get them longer, such as 7". That will allow you to be further from the material…
There is a caveat to this in that all of the beam energy must completely pass through the nozzle…
These are some common lenses and focus distances… notice the lens position in the tube and the size of beam cone as it nears the focal point.
I engrave my down tube with these values… I’m always uncertain 
Make sense…?
Everyone probably does it differently…
Shorter lens (focal distance) smaller dot, less depth of field, longer lens bigger dot and more depth of field.
